Guitarists who play an electric guitar require an instrument amplifier in order to complete the guitar/amplifier system. This “system” is distinguished from a traditional amplifier system such as found in Hi-Fi or sound reinforcement applications in that the system imparts certain characteristics of its own which are considered desirable, even necessary, to the performance of the instrument to the extent that the guitarist (player) is intimately aware of a characteristic “feel” that exists in the response of a guitar amplifier. This “feel” is an important component in a guitar amplifier that can make the musician like or dislike the amplifier and is the result of numerous factors in the amplifier design—specifically tube amplifier design—including the transformers, parts used, tubes selected and numerous other factors.
One critical design element to achieve “feel” is the gain, distortion behavior and low frequency rolloff of the input gain stage amplifier. In a typical guitar amplifier, the rolloff is traditionally set at the first gain stage, commonly referred to as the V1 tube or tube stage. In addition, the bias current of the first stage is an important factor in the gain and sonic behavior of the stage and consequently the amplifier as a whole. In a Class A circuit such as commonly found in the V1 stage, Cathode Biasing is used to set the operating current. Setting the desired bias current and low frequency rolloff entails a combination of a capacitor and resistor connected to the cathode of the relevant V1 tube stage. The bias resistor also serves as the “R” in the R/C time constant equation which sets the low frequency rolloff curve. For example, to get the “Marshall” feel/sound a resistor value of 2.7K ohms and capacitor value of 0.68 microfarads is used. To get the “Fender” feel/sound, a resistor value of 1.5K ohms and capacitor value of 22 microfarads is used.
With reference to FIG. 9 for illustration, a number of bias resistor and rolloff capacitor component value combinations can be derived mathematically or empirically to create different or unique response curves. Over the years, different companies have derived specific combinations of these components in order to achieve their unique sound. Generally speaking, a higher rolloff frequency combined with a lower bias current of the input stage amplifier will yield a “tighter” feel and “smoother” response. This can be interpreted by the guitar player as having a more defined or clearer bass response as well as feeling more responsive or tighter/aggressive to the player. This type of response is generally preferred by musicians performing heavy rock and/or “metal” styles of music. Conversely, a lower rolloff frequency combined with a higher bias current will result in higher fidelity and would be considered “looser” or “fuller” by those skilled in the art. This can be interpreted by a guitarist as a more sluggish or “warmer/vintage” sort of tone which is desirable in many genres outside of heavy rock or metal. This characteristic low frequency rolloff is often crucial to feel and ultimately the satisfaction of the player.
The prior modular instrument amplification system of U.S. Pat. No. 6,376,761 (“the 761 patent”) was introduced to offer players different characteristic and largely recognizable “sounds” by making the modules interchangeable. Many of the modules were intended to be replicates of the gain and sonic profiles of popular traditional amplifiers such as “Marshall,” “Fender” and “Vox” to name a few. Its limitation however was that the bias and low frequency profile of the input gain stage was fixed in order to accommodate a variety of different module configurations. This represented a necessary, but less than optimum compromise. So while the preamp circuitry of each “module” of the '761 patent was different, the input stage behavior did not vary, creating a situation where all modules exhibited similar “feel” which is less desirable to discerning players.